@Christoph-Hart this is how im passing the JSON data:

inline function requestDirectLicenseActivation(deviceName, deviceSignature, accessToken)
    {
        local requestData = {
            "deviceName": deviceName,
            "deviceSignature": deviceSignature
        };
        
        currentAccessToken = accessToken;
        
        makeHttpRequest("POST", "/client/licenses/" + PRODUCT_ID + "/request", trace(requestData), handleDirectLicenseResponse);
    }

Even when converting the JSON object using trace, I'm getting the 415 error due to an unsupported media type, as the endpoint will not receive the content as application/json, as its converted to x-www-form-urlencoded data.